Chapter 5 The LC-3

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-2 Data Movement Instructions Load -- read data from memory to register LD: PC-relative mode LDR: base+offset mode LDI: indirect mode Store -- write data from register to memory ST: PC-relative mode STR: base+offset mode STI: indirect mode Load effective address -- compute address, save in register LEA: immediate mode does not access memory

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-3 PC-Relative Addressing Mode Want to specify address directly in the instruction But an address is 16 bits, and so is an instruction! After subtracting 4 bits for opcode and 3 bits for register, we have 9 bits available for address. Solution: Use the 9 bits as a signed offset from the current PC. 9 bits: Can form any address X, such that: Remember that PC is incremented as part of the FETCH phase; This is done before the EVALUATE ADDRESS stage.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-4 LD (PC-Relative) R Dst <- M[ PC + IR[8:0] ] Note: The ALU is shown performing the add, but there may be a separate adder for address generation.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-5 ST (PC-Relative) M[ PC + IR[8:0] ] <- R Dst

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-6 Base + Offset Addressing Mode With PC-relative mode, can only address data within 256 words of the instruction. What about the rest of memory? Solution #1: Use the value in a register to generate a full 16-bit address. 4 bits for opcode, 3 for src/dest register, 3 bits for base register -- remaining 6 bits are used as a signed offset. Offset is sign-extended before adding to base register.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-7 LDR (Base+Offset) R Dst <- M[ R Base + IR[8:0] ]

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-8 STR (Base+Offset ) M[ R Base + IR[8:0] ] <- R Dst

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-9 Indirect Addressing Mode With PC-relative mode, can only address data within 256 words of the instruction. What about the rest of memory? Solution #3: Read address from memory location, then load/store to that address. First address is generated from PC and IR (just like PC-relative addressing), then content of that address is used as target for load/store.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display LDI (Indirect) R Dst <- M[ M[ PC + IR[8:0] ] ]

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display STI (Indirect) M[ M[ PC + IR[8:0] ] ] <- R Dst

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Load Effective Address Computes address like PC-relative (PC plus signed offset) and stores the result into a register. Note: The address is stored in the register, not the contents of the memory location.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display LEA (Immediate) R Dst <- PC + IR[8:0]

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display LC-3 Data Path Revisited Filled arrow = info to be processed. Unfilled arrow = control signal.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Using Branch and Load Instructions Compute sum of 12 integers. Numbers start at location x3100. Program starts at location x3000. R1  x3100 R3  0 R2  12 R2=0? R4  M[R1] R3  R3+R4 R1  R1+1 R2  R2-1 NO YES

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Sample Program AddressInstructionComments x R1  x3100 (PC+0xFF) x R3  0 x R2  0 x R2  12 x If Z, goto x300A (PC+5) x Load next value to R4 x Add to R3 x Increment R1 (pointer) X Decrement R2 (counter) x Goto x3004 (PC-6)